How fast an appliance transfers energy, the two power equations, changing kW and mA, and choosing the right fuse.
What you will learn
- Calculate power with P = VI
- Calculate power, current or resistance with P = I²R
- Change kW to W and mA to A first
- Choose a 3 A, 5 A or 13 A fuse
Power is how fast energy is transferred
The power of an appliance is the energy it transfers each second.
One watt (W) is one joule per second. For anything in a circuit:
power = potential difference × current
P = V × I
Power in watts (W), p.d. in volts (V), current in amps (A). Recall this one (AQA, Edexcel and OCR).
- A kettle on the 230 V mains takes a current of 10 A
- P = 230 × 10
- P = 2300 W = 2.3 kW
A power rating in kW has to become watts before you use it: 2.3 kW = 2300 W. To find a current from a power, rearrange: I = P ÷ V.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Power is how fast energy is transferred
- Power from current and resistance
- Choosing a fuse
- Try it: Check it with the calculator
- Try it: Your first power calculation
9 practice questions, marked as you go; about 20 minutes in all.
Key terms
Just above
In this unit: Mains electricity, the National Grid and static
Direct and alternating current, the three-core cable and plug, electrical power and energy, the cost of electricity, the National Grid and transformers, and static charge and electric fields.